1. Field of the Invention
The invention disclosed herein is directed to methods and apparatus for cleaning or removing gelled drilling mud, well cuttings and other debris from the internal surfaces of equipment used during the drilling of offshore wells including, but not limited to, subsea blowout preventor assemblies, subsea wellheads and drilling risers. The invention disclosed herein is further directed to methods and apparatus for retrieving wear bushings seated within subsea wellheads. The invention disclosed herein is further directed to methods and apparatus for cleaning the internal surfaces of offshore drilling equipment, as well as the retrieving of wear bushings through the use of a single pipe trip.
2. Description of Related Art
Exploration for, and development of, offshore oil and gas reserves is typically an extremely expensive, high-risk venture. When a fixed platform is already in place, wells can be drilled using a platform-supported drilling rig. However, because of the high cost required to design, fabricate and install a platform and its associated production facilities, oil companies will typically defer this investment until the existence of sufficient oil and gas reserves to justify such an investment have been proved up through drilling operations. As a result, many offshore well are drilled using moveable rigs such as drill ships, semi-submersible rigs and jack-up rigs before a permanent platform is ever installed.
Drilling operations conducted from moveable drilling rigs such as drill ships, semi-submersible rigs and jack-up rigs differ from operations conducted from platform-supported drilling rigs in many respects. Among these differences is the location of the blowout preventor and wellhead assemblies. When drilling from drill ships, semi-submersible rigs and certain jack-up rigs, the blowout preventor and wellhead assemblies are not located on the rig, but rather on the sea floor; as a result, specialized equipment known as "subsea" blowout preventors and wellheads are utilized. A large diameter, flexible pipe known as a riser is used to connect the subsea assemblies to the offshore rig. During drilling operations, drill pipe and other downhole equipment is lowered from the rig through the riser, as well as through the subsea blowout preventor assembly and wellhead, and into the hole which is being drilled.
During drilling operations, including those conducted from moveable drilling rigs, drilling mud is typically pumped down the inside of the drill pipe and circulated up the annulus which is formed between the external surface of the drill pipe and the internal surface of the wellbore. The general functions of drilling mud are: (1) to cool and lubricate the drill bit and downhole equipment during drilling operations; (2) to transport rock cuttings and other debris from the bottom of the hole to the surface; (3) to suspend cuttings and debris during periods when circulation is stopped; (4) to provide hydrostatic pressure to control encountered subsurface pressures; and (5) to seal the hole with an impermeable filter cake. Drilling muds often contain various elements such as gelling agents (e.g. colloidal solids and/or emulsified liquids), weighing materials and chemicals necessary to control properties within desired limits. This drilling mud, as well as cuttings and other debris contained therein, often collect and accumulate on the inner surface of the riser, as well as the subsea blowout preventor assembly and wellhead. This build up of mud, cuttings and debris can significantly impede the drilling process, particularly when casing is being installed in the well.
During drilling operations, a desired length of hole is initially drilled. Thereafter, casing is installed and cemented in the hole. Additional hole is then drilled out of the bottom of the casing using a smaller diameter drill bit and bottom hole assembly. After a desired length of new hole is drilled, an additional string of casing is then installed and cemented in the well. In order to fit concentrically within the well, each successive casing string must be of smaller outer diameter than the inner diameter of the previous casing string which has been installed in the well. The bottom portion of each casing string is typically cemented in place, while the top end of each casing string is seated and secured within the wellhead assembly. When a casing string is seated or installed within a subsea wellhead assembly, it is generally advantageous that a pressure-tight seal is formed between the casing string and the wellhead assembly. In order to facilitate such a seal, the internal surface of the subsea wellhead often includes a polished bore receptacle which is designed to receive the casing string. Generally, very low tolerances exist between the casing and the internal surface of the wellhead assembly. As a result, the internal surface of the wellhead assembly, and particularly the polished bore receptacle, must be clean and relatively free from wear so that each successive casing string can be properly seated and sealed within the wellhead.
Further, the internal surfaces of the drilling riser and blowout preventors must also be relatively free of large amounts of drilling mud, well cuttings and other debris so that these materials are not pushed or scraped into the wellhead assembly when casing is being installed in the well. As each casing string is lowered into the well through the drilling riser, the casing can ride along the inner walls of the riser, and scrape mud and debris off the inner surface of the drilling riser and blowout preventor assemblies. If mud and debris build-up is not removed from the riser and blowout preventor assembly, it can often collect and accumulate at the wellhead, thereby preventing the casing from being properly received within the wellhead assembly.
Additionally, the drilling process itself can also cause wear on the internal surface of the wellhead, thereby damaging the inner profile of said wellhead and making it difficult for casing to be properly received within said wellhead. In order to prevent this wear, as well as the resultant damage that it can create, it is common to temporarily place a sacrificial wear bushing within the wellhead during drilling operations. As drill pipe is rotated and reciprocated in and out of the well, the wear bushing protects the inner surface of the wellhead from wear. Thereafter, once a desired length of hole has been drilled and casing is to be installed in the well, the wear bushing must be retrieved, exposing the inner surface of the wellhead which allows the casing to be seated within the wellhead assembly. Casing cannot be installed in the wellhead without the wear bushing being removed.
Drilling rigs are often "leased" for the length of time necessary to drill a well or wells, and paid for based on a daily rate; as such, the longer drilling operations take, the more expensive the project becomes. Accordingly, it is advantageous to save time and utilize efficient procedures. The tool of the present invention allows the rig operator to wash the drilling riser and subsea equipment, as well as remove the wear bushing, all in a single operation, thereby saving significant amounts of time and money.
The prior art contains numerous tools which utilize mechanical scraping action to clean internal wellbore surfaces, such as casing walls. Examples of such prior art includes U.S. Pat. Nos. 2,275,939; 2,575,307: 4,479,538; 4.809.793; 4,838,354; 4,979,566; 5,076,365; and 5,372,191. Additionally, the prior art contains numerous tools which utilize hydraulic means to clean internal wellbore surfaces, including casing walls. Examples of such prior art includes: U.S. Pat. No. 4,349,073; U.S. Pat. No. Re. 31,495; U.S. Pat. Nos. 4,441,557; 4,781,250; 5,348,086; and 5,564,500. Similarly, the prior art also contains numerous references which describe wear bushing retrieving tools, such as: U.S. Pat. Nos. 3,473,608; 3,645,328; 4,625,381; 4,995,458 and 5,199,495. However, the above references do not disclose an apparatus which can be utilized to clean subsea drilling equipment using both mechanical and hydraulic means, and which can also be utilized to retrieve wear bushings, all in a single pipe trip.
Thus, it is advantageous to have a means to clean and remove gelled drilling mud, well cuttings and other debris from the internal surfaces of drilling risers, subsea blowout preventers and subsea wellhead assemblies. Additionally, it is advantageous to have a means for cleaning said drilling risers, subsea blowout preventors and subsea wellhead assemblies, and further to retrieve wear bushings, through the use of a single pipe trip.